$ \left(\dfrac{49}{81}\right)^{-\frac{3}{2}}$
$= \left(\dfrac{81}{49}\right)^{\frac{3}{2}}$ $= \left(\left(\dfrac{81}{49}\right)^{\frac{1}{2}}\right)^{3}$ To simplify $\left(\dfrac{81}{49}\right)^{\frac{1}{2}}$ , figure out what goes in the blank: $\left(? \right)^{2}=\dfrac{81}{49}$ To simplify $\left(\dfrac{81}{49}\right)^{\frac{1}{2}}$ , figure out what goes in the blank: $\left({\dfrac{9}{7}}\right)^{2}=\dfrac{81}{49}$ so $ \left(\dfrac{81}{49}\right)^{\frac{1}{2}}=\dfrac{9}{7}$ So $\left(\dfrac{81}{49}\right)^{\frac{3}{2}}=\left(\left(\dfrac{81}{49}\right)^{\frac{1}{2}}\right)^{3}=\left(\dfrac{9}{7}\right)^{3}$ $= \left(\dfrac{9}{7}\right)\cdot\left(\dfrac{9}{7}\right)\cdot \left(\dfrac{9}{7}\right)$ $= \dfrac{81}{49}\cdot\left(\dfrac{9}{7}\right)$ $= \dfrac{729}{343}$